1. Field of the Invention
The present invention relates to an electric power steering apparatus that produces a steering assist force by an electric motor.
2. Description of Related Arts
A speed reduction mechanism is used in an electric power steering apparatus for a motor vehicle. Rotation of an output shaft of an electric motor is reduced in speed by, for example, a worm shaft and a worm wheel as the speed reduction mechanism. Thereby, output of the electric motor is amplified and is transmitted to a steering mechanism. As a result, a steering operation is assisted.
The worm shaft and the worm wheel are accommodated in a housing. The worm wheel is together rotatably coupled to an output shaft of a steering shaft coupled to a steering member. The output shaft is rotatably supported by a pair of bearings arranged at both sides sandwiching the worm wheel in an axial direction of the output shaft. The pair of bearings is formed of, for example, roller bearings. Each of the pair of bearings has an inner ring coupled so as to be rotatable together with the output shaft, an outer ring held on the housing, and a rolling element interposed between the inner ring and outer ring (see Japanese Unexamined Patent Publication No. 2005-254864, for example).
However, a gap exists among the inner ring, the outer ring and the rolling elements, and due to the gap, there is a problem that noise occurs as a result of the rolling element colliding against the inner ring and the outer ring during traveling of the motor vehicle.
Therefore, an electric power steering apparatus which eliminates an internal bearing gap by giving an axial preload to the bearing by a screw member and a locking nut has been proposed (see Japanese Unexamined Patent Publication No. 2005-297824, for example).
However, the preloading components such as a screw member and a locking nut are necessary, so that the structure is complicated. For this reason, the number of components and assembly man-hours are increased.
It is an object of the present invention to provide an electric power steering apparatus that can prevent occurrence of noise by a simple structure.